Anti-Flip Approach for Amphibious Aircraft

ABSTRACT

Disclosed are various embodiments for reducing or eliminating the nose-down pitching moment during water landings of amphibious aircraft when the landing gear is in the down position. Shielding struts forward of the wheels generate hydrodynamic lift and reduce hydrodynamic drag in order to alter the pitching moment about the aircraft center of mass.

TECHNICAL FIELD

The present invention relates to the improved safety of amphibiousaircraft during water landings when the landing gear is inadvertentlydeployed in the down position.

BACKGROUND ART

If a pilot of an amphibious aircraft mistakenly deploys the landing gearwheels down in a water landing due to distraction or other reason, theaircraft typically flips over. Sometimes the occupants are trappedwithin the aircraft and drown. Automatic alarms to alert the pilot toreconfigure the wheels have failed to prevent these tragedies. Afteraccidents, some pilots have complained that they could hardlyconcentrate because of a loud alarm going off. About one person a yeardies this way. Even if the occupants all survive, the aircraft isdamaged, and it may pollute the waterway.

Training has not worked to prevent these accidents in the past. Untilhuman nature changes, it seems probable that pilots will continue tomake mistakes in the future. The purpose of the present invention is toprovide as close to a foolproof design as possible to prevent theseneedless deaths.

When a conventional amphibious aircraft with pontoon floats approachesthe water surface with the wheels mistakenly deployed in the downposition, the wheels first pierce the surface. Surprisingly, the dragfrom a surface-piercing strut is proportional to the square of the widthof the strut, according to Hoerner (1965), rather than proportional tothe frontal area of the bluff body. This result is because the drag isassociated with splashing at the free surface. For small depths muchless than about the width of the strut, the drag is independent of thedepth of the surface-piercing strut. As the depth increases, eventuallythe drag will depend on the depth as well as the width of the strut.

A related reference on spray drag is Chapman (1971). The wake or lee ofsurface piercing struts at large Froude number is typically ventilatedby air (Kiceniuk 1954). At sufficiently high speed, there can even becavitation, where the static pressure falls to the vapor pressure ofwater at that temperature and the water boils. For non-steady flow, suchas a sudden encounter with the surface, for example due to waves orrapid descent, virtual mass effects become important (von Karman 1929).

At landing speeds, the large drag force component from thesurface-piercing wheels acts well below the aircraft's center of mass,or “center of gravity” (CG). The result is a large nose-down pitchingmoment about the center of mass. The aircraft typically flips over in aviolent somersault or half somersault. After a severe deceleration,occupants may then find themselves inverted and trapped in a floodingcabin, assuming that they are still conscious.

A nose-wheel support structure for a conventional amphibious nose gearis illustrated in FIG. 1. A large, flat-plate spring is oriented at anegative angle of attack. When it pierces the water surface at landingspeed, a significant negative lift force component as well as a largedrag force component are generated, both of which contribute to a largenose-down pitching moment about the center of mass of the aircraft. Theforce vector from these two components is pointed down and aft,essentially orthogonal to the plate, with a large moment arm about thecenter of mass.

INVENTION

The remedy is simple. Reduce or completely eliminate the nose-downpitching moment from the landing gear piercing the water surface.

While it is not feasible to eliminate the large drag force component ofthe surface-piercing wheels moving at landing speed, it is feasible toreduce their drag component while simultaneously generating asufficiently large lift force component from the nose gear that rotatesthe resultant force vector up, so that it points closer toward theaircraft's center of mass. If that total force vector happens to pointexactly at the center of mass, the pitching moment about the center ofmass from it is exactly zero. The lethal nose-down pitching moment hasthen been eliminated. The lift component can be further increased, sothat the total force vector now points in front of and above the centerof mass to provide a nose-up pitching moment. This may be necessary dueto the main wheels generating a nose-down moment. Too much nose-upmoment may result in an undesirable porpoising motion of the aircraft.The optimal amount of nose-up pitching moment, if any, may depend on thedetails of the aircraft and its floats, and may need to be determinedempirically in taxi and flight tests. A reasonable starting point forthose tests is a zero or slightly nose-up moment.

For both bluff bodies and surface-piercing struts, the skin friction isnegligible. In the absence of appreciable tangential stresses andleading-edge suction, the net force on a plate must be perpendicular tothe plate. Consider a narrow, sloping plate or strut in front of eachnose wheel and its exposed support structure such that the strut isperpendicular to a line that goes through or above the center of mass ofthe aircraft. If the strut completely shields the wheel and itsstructure from the hydrodynamic flow, then the fatal nose-down pitchingmoment is eliminated.

Because the spray pattern downstream of a surface-piercing strut widenswith downstream distance, the drag on a downstream body even somewhatlarger than the strut can be relatively small (Tulin 1957 and Wagner1933). The downstream body is shielded by the upstream one. The upstreamshielding or deflector strut may be narrower that the nose wheel and itsstructure, while still shielding them from the water flow and reducingtheir combined drag. The afterbody may be the spring or other supportstructure of the gear. It may also be the tire and wheel.

The spanwise width of the strut may locally vary, accommodating thevarying local width of the wheel and its support structure. Thenarrowest possible strut is desirable to reduce both its weight and itsaerodynamic drag when the wheels are up.

As the normalized depth increases, the physics changes. The spray at thefree surface no longer dominates the dynamics. At sufficiently largedepth, the wake flow is in a uniform environment (Wu 1972).

The strut may extend as close as practical to the bottom of the tire onthe wheel. Of course, there must be some clearance between the deflectorstrut and the ground when on land. The strut may be composed of areplaceable or flexible segment at its lower end, so that damage to itfrom rocks during ground operations is readily and inexpensivelyrepairable.

An important consideration is yawing moments. If the yaw angle withrespect to the water flow is not zero for whatever reason, there can bea side force at the nose wheels, resulting in a yawing moment, a rollingmoment, or both. If large enough, these moments can cause the aircraftto cartwheel or turn broadside to its direction of motion, resulting insevere deceleration.

In order to inhibit this, the deflector strut may be modified into anon-flat shape in cross section, concave on its upstream face. When theaircraft is yawed, the concave face will preferentially deflect thespray to tend to reduce the yawing angle. For sufficient strength atminimum weight, the deflector strut may be an angle section or modifiedangle section of aluminum or steel.

In another embodiment, the deflector strut may be an angle section withthe apex on the upstream side, so that its forebody drag is minimizedwhile still providing shielding of an afterbody. In the limit of perfectshielding of the afterbody, the afterbody drag would vanish.Consequently, the combined drag for both the forebody and the afterbodywould be just the forebody drag. If the forebody is narrower than theafterbody and if the drag goes as the square of the width, then thecombined drag would be less than that of an isolated afterbody. If thesplash trajectory is straight and tangent to the half angle of theshielding strut, then there is a simple trigonometric relationshipbetween that half angle theta, the width of the afterbody D, and theseparation S between the fore- and the afterbody, tan (theta)=D/2S. HereS is defined as the distance from the upstream apex or virtual apex ofthe forebody to the downstream station of maximum width of theafterbody.

While the force on a surface-piercing strut is initially dominated bythe momentum exchange due to splashing and is thus independent of depth,with increasing penetration depth, eventually the force will also dependon the depth. This transition is expected to occur when the depthbecomes comparable to or greater than the width of the strut, dependingon the geometry of the upstream side of the strut.

The deflector strut need not be exactly flat in the side view. Providedits lift force component is sufficient to raise the net force vector topoint sufficiently close to or above the center of mass, the aircraftwill not flip over.

Retrofits to existing amphibious aircraft may simply add the deflectorstrut to an existing aircraft or modify the spring bar so that it hasthe appropriate slope. For new, clean-sheet designs, the deflector strutcan be incorporated into the support structure of the nose wheel, suchas the spring bar.

Note that the addition of the deflector strut can decrease the dragcomponent over that of the gear assembly alone. The drag of asurface-piercing strut goes as the square of the spanwise width forshallow depths. If a shielding strut in front of the nose gear isnarrower than the gear assembly, the total drag may be less that withoutthe shielding strut. An optimal deflector strut would have the minimumsize necessary to generate the required lift and drag components.

The main gear also generates a nose-down pitching moment. Since they areslightly aft of the aircraft CG, it is not feasible to generate anose-up pitching moment with positive lift from deflector struts intheir vicinity. In principle, a negative-lift deflector strut near themain gear might assist in generating a nose-up pitching moment. However,the CG is only slightly forward of the main gear, so the availablemoment arm for the lift component is small. Properly sized to shield themain gear from the hydrodynamic flow, a small deflector strut wouldreduce the drag component of the main gear and the associated nose-downpitching moment. It may also rotate the total force vector to reduce themoment arm about the center of mass and hence the nose-down pitchingmoment. Finally, the drag component of a shield strut leaning forwardmay be less than that of a vertical strut.

Note also that the hydrodynamic forces of this invention are equallyeffective at reducing the tendency to flip during downwind landings. Thebeneficial change in pitching moment is generated purely by the dynamicpressure of the hydrodynamic flow past the shielding struts and thelanding gear. At fixed airspeed, the dynamic pressure of the water flowbecomes relatively stronger during a downwind water landing. Theinvention does not rely on the effectiveness of the aircraft'saerodynamic surfaces, which become relatively weaker in a downwind waterlanding.

While the original invention was motivated by the tragic deaths inamphibious float planes with retractable land wheels, the benefits ofthe invention are available to amphibious aircraft with fuselage hullsrather than pontoon floats. This is also true for amphibious aircraftwith non-retractable land wheels. In that case the deflector struts maybe non-retractable as well.

DESCRIPTION OF THE DRAWINGS

In FIG. 1, a conventional amphibious aircraft 1 with floats 2 lands onthe water surface 3 with the nose wheels 4 and main wheels 5 mistakenlyin the down position. Force vector 6 is generated on the nose wheelsupport structure 7, and force vector 8 is generated on the main wheels,resulting in a large nose-down pitching moment 9 about the aircraftcenter of mass 10.

In FIG. 2, deflector struts 11 have been incorporated into the supportstructure forward of nose wheels 4 and support structure 7 to generateforce vector 12, along a line 13 that is oriented forward of theaircraft center of mass 10, resulting in a nose-up pitching moment 14.Deflector struts 15 have been incorporated forward of the main wheels 5to generate force vector 16, along a line 17.

In FIG. 3, deflector strut 11 is mounted forward of nose wheel 4 and itssupport structure 7.

In FIG. 4, illustrates a concave geometry of the upstream surface 19 ofa deflector strut 20, positioned upstream of nose wheel 4 in ahorizontal cross section below the wheel axle.

In FIG. 5, a horizontal cross section below the wheel axle illustrates awedge geometry of the upstream side 21 of a deflector strut 22, withhalf angle 23, positioned upstream a distance 24 forward of nosewheel 4of width 25.

REFERENCES

Chapman, R. B. 1971 Spray Drag of Surface-Piercing Struts, TP251, NavalUndersea Research and Development Center, September.

Hoerner, S. F. 1965 Fluid-Dynamic Drag, self-published.

Kiceniuk, T. 1954 A Preliminary Experimental Study of VerticalHydrofoils of Low Aspect Ratio Piercing a Water Surface, Report No.E-55.2, Hydrodynamics Laboratory California Institute of Technology,Pasadena, Calif.

Tulin, M. P. 1957 David W. Taylor Model Basin, Washington, D.C., USA,Department of the Navy, Published in: Schiffstechnik, Band 4, Heft 21.

von Karman, T. 1929 The Impact on Seaplane Floats during Landing,National Advisory Committee for Aeronautics, 321 309-313.

Wagner, H. 1933 Über das Gleiten von Wasserfahrzeugen, Jahrbuch derSchiffbautechnik,” 34. Also published in English as NACA TM 1139.Washington, April 1948.

Wu, T. Y-T. 1972 Cavity and wake flows, Annual Reviews of FluidMechanics, 4, 243-284.

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of various embodiments of theinvention. In this regard, no attempt is made to show structural detailsof the invention in more detail than is necessary for the fundamentalunderstanding of then invention, the description taken with the drawingsand/or examples making apparent to those skilled in the art how theseveral forms of the invention may be embodied in practice.

As used herein and unless otherwise indicated, the terms “a” and “an”are taken to mean “one”, “at least one” or “one or more”. Unlessotherwise required by context, singular terms used herein shall includepluralities and plural terms shall include the singular.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words ‘comprise’, ‘comprising’, and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to”. Words using the singular or pluralnumber also include the plural and singular number, respectively.Additionally, the words “herein,” “above,” and “below” and words ofsimilar import, when used in this application, shall refer to thisapplication as a whole and not to any particular portions of theapplication.

The description of embodiments of the disclosure is not intended to beexhaustive or to limit the disclosure to the precise form disclosed.While the specific embodiments of, and examples for, the disclosure aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the disclosure, as thoseskilled in the relevant art will recognize.

All of the references cited herein are incorporated by reference.Aspects of the disclosure can be modified, if necessary, to employ thesystems, functions, and concepts of the above references and applicationto provide yet further embodiments of the disclosure. These and otherchanges can be made to the disclosure in light of the detaileddescription.

Specific elements of any foregoing embodiments can be combined orsubstituted for elements in other embodiments. Moreover, the inclusionof specific elements in at least some of these embodiments may beoptional, wherein further embodiments may include one or moreembodiments that specifically exclude one or more of these specificelements. Furthermore, while advantages associated with certainembodiments of the disclosure have been described in the context ofthese embodiments, other embodiments may also exhibit such advantages,and not all embodiments need necessarily exhibit such advantages to fallwithin the scope of the disclosure.

What is claimed is:
 1. A deflector strut system configured in front ofeach nose wheel of an amphibious aircraft to reduce or to eliminate thenose-down pitching moment during wheels-down water landings, the systemcomprising a shield or deflector strut positioned forward of each nosewheel when the nose gear is in the down position.
 2. The system of claim1 wherein the deflector strut is narrower in the spanwise direction thanthe nose wheel and tire.
 3. The system of claim 1 wherein the deflectorstrut has a non-constant spanwise width.
 4. The system of claim 1wherein the deflector strut has a concave upstream surface.
 5. Thesystem of claim 1 wherein the deflector strut has a convex upstreamsurface.
 5. The system of claim 1 wherein the perpendicular bisector tothe deflector strut from its centroid is a line that extendssubstantially through the center of mass of the aircraft.
 6. The systemof claim 1 wherein the perpendicular bisector of the deflector strutfrom its centroid is a line that extends in front of the center of massof the aircraft.
 7. The system of claim 1 wherein the deflector strut iscurved.
 8. The system of claim 1 wherein the lower extremity of thedeflector strut consists of a replaceable segment.
 9. The system ofclaim 1 wherein the deflector strut is incorporated into the nose wheelsupport structure.
 10. The system of claim 1 wherein the deflector strutis a spring for the nose wheel.
 11. The system of claim 1 wherein thetop of the deployed deflector strut is forward of the bottom of thedeployed deflector strut.
 12. The system of claim 1 wherein thedeflector strut is deployed in the down position by means of mechanicalconnection to the nose gear.
 13. A deflector strut system configured infront of each main gear wheel of an amphibious aircraft to reduce thehydrodynamic drag during water landings, the system comprising: adeflector strut; and structure to support the strut.
 14. The system ofclaim 13 wherein the spanwise width of the deflector strut is less thanthe width of the main gear wheel and tire assembly.
 15. The system ofclaim 13 wherein the top of the deflector strut is forward of the bottomof the strut.
 16. The system of claim 13 wherein the deflector strut ismechanically connected to the main gear.